Beginning in the post-war construction boom of the late 1950s and early 1960s, overhead electrical cable lines were recognized as an eyesore. Underground electrical cable technology was developed and implemented due to its aesthetic advantages and reliability. Underground electrical cable, a medium voltage cable that carries from 5,000 volts to 35,000 volts with an average voltage of 15,000 volts, initially employed high molecular weight polyethylene (HMWPE) polymer as the insulation of choice due to its low cost and ease of manufacturing. Subsequently, cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR) replaced high molecular weight polyethylene as the insulation. More recently, a water damage retardant formulation has also been included in these newer types of insulation.
Underground electrical cable was initially touted as having a useful life of from 25 to 40 years. However, the useful life of underground cable has rarely exceeded 20 years, and has occasionally been as short as 10 to 12 years. Catastrophic failure of older HMWPE, XLPE, and EPR cable is now beginning to occur due to water damage known as "water trees." Water trees are formed in the polymer when medium to high voltage alternating current is applied to a polymeric dielectric (insulator) in the presence of liquid water and ions. As water trees grow, they compromise the dielectric properties of the polymer until the insulation fails. Many large water trees initiate at the site of an imperfection or contaminant, but contamination is not a necessary condition for water trees to propagate.
Water tree growth can be eliminated or retarded by removing or minimizing the water or ions, or by reducing the voltage stress. Voltage stress can be minimized by employing thicker insulation. "Clean room" manufacturing processes can be used to both eliminate ion sources and minimize defects or contaminants that function as water tree growth sites. Another approach is to change the character of the dielectric, either through adding water tree retardant chemicals to polyethylene or by using more expensive, but water tree resistant, plastics or rubbers. All of these approaches have merit, but only address the performance of electrical cable yet to be installed.
For electrical cables already underground, the options are more limited. First, the entire failing electrical cable can be replaced, but the cost is often prohibitive. Second, the points of failures due to water tree propagation can be excised and the removed portions replaced with a splice. Unfortunately, since water trees are not identifiable until after cable failure occurs, splicing after cable failure results in a power interruption to the electric utility customers. Third, the cable can be dried with a desiccant fluid such as nitrogen in order to remove the water that initiates the water tree. While this approach improves the dielectric properties of the underground cable, it requires perpetual maintenance to replace large and unsightly nitrogen bottles that remain coupled to the cable.
A more promising approach to retard failure of underground cable is to inject a silicone fluid such as, for example, CABLECURE.RTM., into the electrical cable conductor strands. CABLECURE reacts with water in the underground cable and polymerizes to form a water tree retardant that is more advanced than those used in the manufacture of modern cables. The dielectric properties of the cable are not only stabilized by CABLECURE, but actually improved dramatically.
However, the devices and methods used to treat underground electrical cables with CABLECURE do have drawbacks. Different methodologies are employed depending upon the type of cable being treated. There are two main classes of cables, underground residential distribution (URD) cables which are relatively small cables, and feeder cables, which are larger cables which often supply the URD cables.
Regarding the treatment of feeder cables with CABLECURE, a major problem is the ability of splices which are often encountered in the feeder cable to hold the pressure required to inject perhaps miles of the feeder cable with CABLECURE. The larger the overall cable diameter, the larger the splice, and the higher the hoop forces created by the pressurization of the cable cavity. Due to the large diameter of feeder cables, there is seldom sufficient hoop strength in the typical splices to withstand the basic vapor pressure of the CABLECURE without leaking, not to mention the increased pressurization required to transport the CABLECURE along the miles of feeder cable. A leak of CABLECURE in the splice can create a contaminated path along the splice interface which may lead to eventual failure of the splice.
To avoid the problem of CABLECURE leaking at splices, one of two approaches have been employed for injection of CABLECURE into feeder cables. First, the splice can be reinforced with clamps or other devices to increase its hoop strength. However, this approach is limited because the force necessarily applied by the hose clamps or other reinforcement devices on the splice is so large that there is substantial deformation of the rubber material used to make the splice. The deformation compromises the geometrical and electrical integrity of the splice and thus provides only a slight increase in injection pressure tolerance. A second approach is to remove the splice prior to injecting the two separated segments of the electrical cable with CABLECURE, then injecting CABLECURE, and finally injecting a second damming chemical compound into the two electrical cable segments that physically blocks the migration of the CABLECURE into a new splice that is applied to the two cable segments after the CABLECURE treatment has been completed. An example of a damming compound is a combination of dimethylsilicone polymers with vinyl cross-linker and a suitable catalyst. In addition to low viscosity and quick cure times, a damming fluid must be compatible with all cables, splices and other components. Drawbacks with the above method of employing a damming compound include the additional cost of the expensive damming compound, the necessity to install a new splice, and the possibility that the CABLECURE may compromise the structural integrity of the new splice if the physical partition formed by the damming compound fails.
Further, it has been learned that injection of damming compounds into even short lengths near the end of a cable can create transient discontinuities in the penetration of the dielectric enhancement fluid. These discontinuities of penetration create discontinuous treatment, which at a minimum leaves some small section of the cable untreated for a longer period of time, increasing the risk of a post treatment dielectric failure. Further, there is a potential that these discontinuities can even lead to local electrical stress increases which may contribute to a failure in the region where the dam interferes with uniform penetration. Since the point of injecting cable is to increase its reliability and mitigate its proclivity to fail, the use of either reinforcing devices or damming compounds to handle sufficient injection, vapor and elevation-induced pressure are not ideal solutions.
CABLECURE injection can also be employed to treat water tree damage in URD cables. Since the diameter of the URD cables is less than that of feeder cables, the splices in URD cables can withstand the vapor pressure of CABLECURE. Additionally, due to the typically shorter lengths of the URD cables, a lower pressure (0-30 psig) than the pressure employed in feeder cables is required to transport the CABLECURE through the URD cable; therefore, the splices in the URD cable are not subjected to the moderate pressures (30-120 psig) desired to inject typically longer feeder cable and their integral splices. However, because an URD cable does not have enough interstitial volume in the strands of the cable to hold sufficient CABLECURE for maximum dielectric performance, URD cables require an extended soak period of 60 days or more to allow for additional CABLECURE to diffuse from the cable strands into the polyethylene. When very long URD cables or URD cables with large elevation changes are encountered, moderate to medium (120-350 psig) pressure injection of CABLECURE may be required. The moderate to medium pressure addition of CABLECURE to an URD cable therefore necessitates removing the splices during the treatment of the cable, followed by adding new splices after the treatment.
A need thus exists for devices and methods whereby expensive damming compounds are not required to block the contact of repair chemicals with the replacement splice in feeder cables.
A need also exists for devices and methods in which both a separate conduit for injecting CABLECURE into a feeder cable as well as a separate replacement splice are not required.
A further need exists for devices and methods in which repair chemicals can be injected into URD cables at moderate to medium pressures without compromising the structural integrity of splices.